This invention relates to an improvement in the compression of delta coded sequences representative of successive scan lines of a continuous tone image.
In the prior art, J. S. Wholey (IRE Transactions on Information Theory, April 1961, pages 99-104) pointed out that compression would result if the binary threshold-encoded elements of successive scan lines of graphical images were applied to a predictive encoder. Wholey further pointed out, at page 100, that a run-length code could always be found for coding the predictor output. It is to be recalled that the generic predictive encoding compression technique was introduced by P. Elias (IREa Transactions on Information Theory, March 1955, at pages 16-33). Relatedly, R. E. Graham in U.S. Pat. No. 2,905,756, "Method and Apparatus for Reducing Television Bandwidth" Sept. 22, 1959) applied predicitive encoding to the threshold-encoded elements of successive television scan lines. The term "threshold encoding" was taken to mean resolving each picture element independently as having either a black or a white picture value.
The question naturally arises as to how may one encode images having tones between black and white. If a scan represents continuum of values, then differential encoding can be used. Relatedly, Armin H. Frei in "An Adaptive dual mode Coder/Decoder for Television Signals", IEEE Trans. on communications Technology, December 1971, pages 933-944, points out at page 934 that early attempts to encode video information consisted in sampling the video signal at the Nyquist rate and in coding 64 gray levels by means of 6 binary digits per pel. Later, differential PCM was introduced and where the video waveform was relatively smooth, it was feasible to operate a coder in the delta mode producing one bit per pel.